Increasing emphasis has been placed in recent years upon finding ways to efficiently produce fuels from renewable, non-petroleum resources. In one field of interest, fuel ethanol has been produced by fermentation of biomass feedstocks derived from plants. Currently, fuel ethanol is commercially produced from feedstocks of cornstarch, sugar cane and sugar beets. These materials, however, find significant competing uses in the food industry, and their expanded use to make fuel ethanol is met with increased prices and disruption of other industries. Alternative fermentation feedstocks and viable technologies for their utilization are thus highly sought after.
Lignocellulosic biomass feedstocks are available in large quantities and are relatively inexpensive. Such feedstocks are available in the form of agricultural wastes such as corn stover, corn fiber, wheat straw, barley straw, oat straw, oat hulls, canola straw, soybean stover, grasses such as switch grass, miscanthus, cord grass, and reed canary grass, forestry wastes such as aspen wood and sawdust, and sugar processing residues such as bagasse and beet pulp. Cellulose from these feedstocks is converted to sugars, which are then fermented to produce the ethanol.
FIG. 1 shows a prior known process for producing ethanol from a lignocellulosic biomass starting material. The process is carried out in a sequential fashion. Each step must be completed before the next step can take place. In the first step 24, biomass solids 22 and aqueous sulfuric acid 20 are mixed and pretreated to facilitate subsequent hydrolysis of the biomass. The mixture proceeds to step two 26 where the hemicellulose is hydrolyzed. After the hemicellulose hydrolysis, the mixture is ready for step three 32 where base 31 is added to neutralize acid and adjust pH followed by addition of an enzyme 30 for the hydrolysis of cellulose to give simple sugars. The mixture of 32 after the hydrolysis reaction in complete will be a solution containing glucose and xylose from the biomass. This solution is then moved onto step four 38 where yeast 39 is added to ferment the sugars present in the solution to ethanol. After the fermentation is complete, the ethanol can be recovered 40 from the process solution by, for example, distillation.
One problem with this prior known process is that acids such as dilute sulfuric acid or other mineral acids are used, but these acids cause degradation of materials in the biomass to form substances that can act as inhibitors in subsequent enzymatic and fermentation steps. Material produced by sulfuric acid hydrolysis of the biomass would need to be purified before subsequent enzymatic and fermentation steps or larger amounts of enzyme or yeast would need to be used to overcome the inhibitors that would be present if no purification was done. Either way would increase the cost of carrying out the process.